1. Field of the Invention
The present invention relates to a vehicle power transmission system and more particularly to a power transmission system comprised of a hydraulic torque converter and a multiple-stage gear mechanism arranged in this order coaxially with respect to the axis of the engine crankshaft.
2. Description of the Prior Art
Conventional automatic power transmission assemblies for vehicles, particularly for engine-driven vehicles, generally include a torque converter having an impeller adapted to be connected with the engine crankshaft to be driven thereby, and a turbine which is connected with a multiple stage gear mechanism, such as a planetary gear mechanism provided with clutches and brakes for appropriately selecting one of several gear trains for obtaining a desired gear stage. In order to obtain a desired number of gear stages, for example, three or four gear stages, the planetary gear mechanism usually includes two or more planetary gear units which are arranged in series along the axis of the transmission assembly. The conventional transmission assembly has therefore a substantial longitudinal length and this makes it difficult to adopt this type of transmission in a front engine, front wheel drive type vehicle.
It has recently been recognized that in vehicles of front engine, front wheel drive type, it is advisable to arrange the engine with the crankshaft directed transversely of the vehicle body. Since the vehicle body has a limited transverse width, the transmission assembly is required to have an axial length which is as small as possible. However, the aforementioned conventional transmission cannot be designed to have a sufficiently small axial length.
In Japanese laid open patent application 57-1848, laid open to public inspection on Jan. 7, 1982 and corresponding to the U.S. patent application Ser. No. 149,060, filed on May 12, 1980, now U.S. Pat. No. 4,400,998, there is disclosed a vehicle transmission having a torque converter and a four-stage gear mechanism with a decreased axial length. According to the proposal by the laid open patent application, the gear mechanism comprises a composite planetary gear unit of the so-called Ravigneaux type having a pair of sun gears of different diameters, one or more first planetary pinions meshing with the smaller sun gear, one or more second planetary pinions meshing with the first planetary pinions and the larger sun gear, and a ring gear meshing with the second planetary pinions. A third-overdrive clutch is provided between the torque converter and the gear mechanism whereas a reverse lock one-way clutch and a low-second-third clutch are arranged in this order at an end axially opposite to the torque converter with respect to the gear mechanism. A disc type low-reverse brake and a disc type second-overdrive brake are arranged so as to overhang the gear mechanism, and a reverse clutch is at an axially outward position of the second-overdrive brake. An output gear is located between the torque converter and the third-overdrive clutch. The transmission assembly as proposed by the Japanese laid open application has a small axial length and is therefore considered as being suitable for use in a vehicle of front engine, front wheel drive type. It should however be noted that the low-reverse brake and the second-overdrive brake, which are arranged so as to overhang the gear mechanism, and the reverse clutch, which is arranged at an axially outward portion of the second-overdrive brake, unavoidably increase the diameter of the transmission assembly. It should further be noted that the proposed arrangement has no room for providing further elements such as clutches and one-way clutches which may be required for suppressing gear shifting shocks and decreasing gear shifting time.